Polymers such as polypropylene are vastly applied in end-use products, including automotive applications, packaging applications, house ware applications, storage applications, and the like. There is a general need for an improved visual appearance of clarified polymers with reduced haze and low yellowness.
The use of nucleating agents to reduce the haze in articles prepared from polypropylene compositions is known in the art. Nucleating agents provide nucleation sites for polymer crystal growth during molding or fabrication processes. Compositions comprising nucleating agents typically crystallize at higher crystallization temperatures and at faster rates compared to compositions without nucleating agents.
The efficiency of a nucleating agent is typically measured by the peak crystallization temperature of the polymer compositions. A high polymer peak crystallization temperature is indicative of high nucleation efficacy, which usually translates into faster nucleation rates and thus shorter processing cycle times.
Generally, the presence of many nucleation sites associated with adding a nucleating agent results in a larger number of smaller crystals. As a result of the smaller crystals formed therein, clarification of articles prepared from the polymer may also be achieved, although excellent clarity is not always the result. For applications that require excellent clarity of the polymer article, an additive that induces low haze within the final product is added to the polymer composition. Such compounds are generally called clarifying agents or clarifiers.
A combination of high nucleation efficacy and high article clarity is desirable. An effective clarifying agent known in the state of the art are acetals of sorbitols, such as dibenzylidene sorbitol acetal derivatives. However, acetals of sorbitols exhibit a poor solubility in polypropylene compositions and process temperatures of more than 220° C. are required to dissolve sufficient amounts. Furthermore, the presence of other additives required to adjust the properties of the polypropylene composition according to need influence the efficiency of the clarifying agent and the performance of the other additives.
It is an object of the present invention to provide a polypropylene composition with excellent visual appearance allowing to prepare articles from such polypropylene compositions with equally good to excellent visual appearance.
It has been surprisingly found that the addition of zinc fatty acid salt results in a polymer composition comprising polypropylene with excellent visual appearance suitable for the preparation of articles that also show this favorable visual appearance.